1. Field of the Invention
The present invention relates to an optical disk device, an optical disk track identification method and a method for accessing an optical disk as for instance applicable to an optical disk unit. This invention utilizes a simple configuration to access a specific track by means of a track identification method based on changes in address information from a specified number of track jumps.
2. Description of Related Art
In the method of an optical disk device of the conventional art, address information is held in common on adjacent tracks, allowing tracks to be formed close together and the recording density to be improved.
An optical disk of this type is shown by a plan view for an optical magnetic disk in FIG. 7. An magneto-optic disc 1 is formed with two pre-groups G1 and G2 in a helical shape on the information recording surface. The pre-groups with the names G1 and G2 respectively form laser beam guide grooves. These laser beam grooves allow assigning tracks respectively to the pre-groups G1 and G2 or assigning tracks respectively between the lands of the continuous pre-groups G1 and G2 for thermo-electric recording of the desired data.
Of the two pre-groups G1 and G2, the pre-group G2 is formed to oscillate and the frequency of this oscillation is made to shift in compliance with the address information from a specified center frequency. This center frequency shift allows one piece of address information to be assigned to the No. 1 and No. 2 tracks corresponding to the two pre-groups G1 and G2 and this one piece of address information is held in common by two tracks.
When the pre-groups oscillate and the track density of the recorded address information is improved, cross talk which occurs across adjacent tracks is difficult to avoid and replaying of the correct address information becomes difficult. In contrast, if one piece of address information is held in common by two tracks, then effects of cross talk from adjacent tracks can be effectively prevented and the correct address information can be replayed. This common holding of information therefore permits improved track density and reliable access of the desired track.
However, when utilizing an optical pickup to access the desired track on this type of optical disk which shares one piece of information among two tracks, a track identification means is necessary to access one of these tracks.
To provide such a track identification means, methods were proposed such as the three-spot method in which along with issue of a laser beam, the return of the laser beam was received and address information detected from the pre-group on either the inner circumference or the outer circumference.
However, this method had a problem in that the overall or partial structure of the mechanism for identifying the track was too complex.